Kelly started it, then Hammer and Drew chimed in. I almost wrote something myself yesterday, but then decided to sleep on it since I often do my best thinking while I'm asleep.
"Common sense": Those of us who believe we have it know it when we see it, but how can it be defined, and why do some people have it and some don't?
Upon awakening this morning, an answer--perhaps--had come to me, and it sounds pretty good, so I'll go with it.
Those people who have "common sense" are those people who have accepted intellectual responsibility for their own thought processes.
Accepting responsibility for one's own thought processes spills over into accepting other kinds of responsibility for other things--it's a chain reaction. So if I had to explain the above statement, I would say that it describes those who believe and behave the way they do because they have decided themselves that it is the best way to go--not because someone else told them to: someone else who they can blame for making a bad decision for them.
I would not be surprised at all if it were revealed that there is a vast conspiracy, a sort of anti-Bene Gesserit who are dedicated to breeding the stupidest person in the history of humankind. A person who won't remove his hand from the box, not because he has the willpower to overcome his own instincts against pain, but because he thinks someone else should remove his hand for him.
That's my take on it, anyway.